The present invention relates, in general, to a high frequency (HF) radar method for estimating the radar cross-section of the sea surface and, more particularly, to a method for estimating the HF radar cross-section of ships and aircraft using Doppler spectral properties.
The doppler spectrum of HF (high frequency) radar backscatter from the sea is of particular interest because of its demonstrated potential as a comprehensive remote source of sea surface conditions over large areas of the earth's ocean surface. Operating in a band of frequencies roughly 3-30 MHz, HF radar energy is reflected from the ionosphere back to earth covering ranges, nominally, from 1000 to 3500 km from the radar, with sufficient energy density to be useful for remote sensing. A fraction of this energy is reflected from the sea surface back to the radar with phase and amplitude altered. With doppler processing of such data, information about the directional sea spectrum averaged over the radar cell can be inferred from characteristics in the processed radar power spectrum. Direction and magnitude of the winds exciting the sea surface within the scattering patch can then be inferred using inversion techniques based upon models of the sea spectrum.
The radar scattering coefficient of the sea surface at HF, .sigma..degree., is generally difficult to obtain by a straight-forward well-calibrated radar measurement because several of the variables used in the radar equation are not well known. For either surface wave or skywave illumination of the sea surface, the gain to be used in the radar equation is difficult to estimate because it can be a complex function of local ground, soil moisture, ground screen quality, coupling to the sea surface, tidal effects, and so on. For the case of a skywave measurement the problem is further compounded by the variation of the gain with elevation angle, as well as the ionospheric propagation losses the signal suffers within the ionosphere in its two refractive passes.
Knowledge of the scattering coefficient .sigma..degree. of the sea surface by a means independent of the radar equation would allow the unknown discussed above to be determined as well as prove useful for comparison with targets which might be detected in the Doppler spectrum being measured. From this information, attempts could be made to apply target identification techniques to the target of interest.
Several target identification techniques have been developed over the past several years and are particularly applicable to the HF band of radar frequencies. Some examples of these target identification techniques have been reported by H. C. Lin and A. A. Ksiensky, "Optimum Frequencies for Aircraft Classification", Ohio State University Electroscience Laboratory Technical Report 78 3815-6, 1979. These techniques depend upon a multi-parameter measurement of a target and a comparison of that measurement with a catalog of values of the radar cross-section as a function of the parameter available. The parameters in question typically include radar frequency, polarization and phase. For ionospheric propagation, radar frequency can be varied over a reasonable bandwidth for coverage of a given illuminated area, of the order of 3 to 6 MHz. For the elliptically polarized wave which exits the ionosphere, a measurement of the difference between the minima and maxima in signal amplitude can be used to draw some conclusions about the ratio of horizontal to vertical radar cross-section of a target. Phase information in an absolute or even relative sense between two radar frequencies is not known for ionospheric propagation. Hence, with some information about two of the three parameters available, target identification with skywave HF radar might be possible with a satisfactory calibration cross-section for comparision, such as that of the sea suface.
Another application of such cross-section information is that of remote sensing of the sea surface. It is well known that, to first order, the cross-section of the sea surface is proportional to the components of the ocean wave directional energy spectrum which are traveling toward and away from the radar bearing. Hence, assuming the directional spreading of the wave spectrum is known or can be estimated, a determination may be made as to how highly the sea surface is developed. Coupled with knowledge of the largest wavelengths excited and an estimate of wave spreading with angle, a good estimate of the root mean square wave height may be obtained.
Heretofore, there was no way to accurately estimate the sea surface scattering coefficient and as a result there was no way to estimate the radar cross-section of sea scatter, a related quantity. The radar cross-section has usually been assumed to be a constant value of -29 dbm.sup.2. This assumes that the sea surface is fully developed and does not account for the directional properties of ocean waves. As a result of these assumptions, errors as high as 20 db or more can occur because of uncertainties in ionisphere losses and other propagation phenomena, so that target identification with HF radar has not been feasible using prior art methods.